Distribution of contents such as videos and audios by utilizing networks such as the Internet has been widely spread. In order to continue distribution with stable videos and audios, it is important to properly set the bit rate and the like of the content data transmitted to a receiving terminal from a content distribution device (server) in accordance with the network state that changes moment by moment.
Thus, as depicted in Non-Patent Document 1, a content distribution device regularly transmits probe packets to a reception device, estimates the network state from variations in round-trip time of the probe packets, and sends out content data with a bit rate suited for the estimated network state.
Further, as related patent documents thereof, there are following documents. Among those, depicted in Patent Document 1 is a technique which puts terminals of close distances (small hop number between the terminals) into a group when distributing content from a distribution server to the terminals with multicast, and determines the bit rate for distribution by predicting the useable band by each group.
Depicted in Patent Document 2 is a multicast data distribution method which checks a band width that can be reserved in all communication paths of all receiving terminals prior to transmission of content data, and transmits data by conforming to the band width. Depicted in Patent Document 3 are stream data distribution and the like, with which a capable band width of the communication paths is estimated by using a test stream. Depicted in Patent Document 4 is a technique which enables measurements of performance of IP network using the probe packets to be more precise.    Patent Document 1: Japanese Unexamined Patent Publication 2005-167675    Patent Document 2: Japanese Unexamined Patent Publication 2000-286845    Patent Document 3: Japanese Unexamined Patent Publication 2008-278207    Patent Document 4: Japanese Unexamined Patent Publication 2007-533215    Non-Patent Document 1: “A bit rate control method using round-trip time of probing packets”, Nihei et al., B-6-99, IEICE General Conference 2010
However, with the technique depicted in Non-Patent Document 1 which estimates the network state by the probe packets, it is necessary to transmit a defined number of probe packets to all the receiving terminals that receive the content transmitted from the content distribution device.
Therefore, the number of terminals is increased. Thus, when content is distributed to the great number of terminals simultaneously, the load imposed upon transmission and reception of the probe packets to be transmitted becomes increased accordingly. This causes increase in the load imposed upon the content distribution device, the network apparatus, and the lines, while causing decrease in the number of distributable streams as well as deterioration in the communication quality of each receiving terminal at the same time.
The network state between the distribution device and the receiving terminal is mainly determined depending on the existence of the bottle-neck link (the link whose useable band is the minimum) on the communication path. The bottle-neck link and the useable band fluctuate as the time passes depending on the traffic of a specific terminal and the communication state and the like of a specific communication path. Thus, the time variations in the bit rates of the terminals whose bottle-neck link is the same often show similar tendencies.
Therefore, when the terminals of the same bottle-neck link can be put into a same group, the bit rates of the terminals belonging to the same group can be controlled collectively. Thus, it is considered that the number of probe packets to be sent out can be reduced. However, the technique depicted in Patent Document 1 is designed to put the terminals with the “hop number” of less than a prescribed value, and it is not by any means designed to put the terminals of the same “bottle-neck link” into a same group.
Therefore, when a group is constituted by the method depicted in Patent Document 1, there is a possibility that the terminals of different bottle-neck links are put into a same group. In such case, a proper control of bit rate done collectively for the same group cannot be achieved. For example, there may be a case where the bit rate is suppressed to the extent of more than it is required even though there is a margin in the band or there may be a case of vice versa. Inversely, there may be a case where the terminals of same bottle-neck link are put into different groups with this method. In such case, the effect of reducing the number of probe packets cannot be acquired.
In the rest of Patent Documents 2 to 4, the technique of collectively controlling the bit rates and the like for the terminals of the same bottle-neck link is not depicted. Therefore, the above-described issues cannot be overcome with the techniques depicted therein.
It is an object of the present invention to provide a content distribution system, a content distribution device, a content distribution method, and a program thereof, which can reduce the number of probe packets to be transmitted towards the receiving terminals from the distribution device while properly controlling the bit rates for each of the receiving terminals and thereby can suppress the reduction in the number of streams that can be distributed simultaneously as well as the deterioration in the communication quality.
In order to achieve the foregoing object, the content distribution system according to the present invention is a content distribution system constituted with a content distribution device and a plurality of receiving terminals connected mutually via a network, and the content distribution device transmits content data to the receiving terminals. The content distribution device is characterized to include: a content packet transmission unit which transmits the content data to the receiving terminals; a probe packet transmission unit which transmits probe packets for estimating a state of the network to the receiving terminals; a probe packet reception unit which receives return packets for the probe packets from the receiving terminals; and a transmission control unit which specifies round-trip time from transmission of the probe packets to reception of the return packets, and determines a transmission interval of the probe packets and the bit rate of the content data to be transmitted from the round-trip time; and the transmission control unit is characterized to define the reception terminals having similar time variations in the bit rate among the receiving terminals as a small group, and to determine the transmission interval of the probe packets and the bit rate of the content data to be transmitted collectively for the receiving terminals belonging to the same small group.
In order to achieve the foregoing object, the content distribution device according to the present invention is a content distribution device which is constituted by being mutually connected to a plurality of receiving terminals via a network and transmits content data to the receiving terminals. The content distribution device is characterized to include: a content packet transmission unit which transmits the content data to the receiving terminals; a probe packet transmission unit which transmits probe packets for estimating a state of the network to the receiving terminals; a probe packet reception unit which receives return packets for the probe packets from the receiving terminals; and a transmission control unit which specifies round-trip time from transmission of the probe packets to reception of the return packets, and determines a transmission interval of the probe packets and the bit rate of the content data to be transmitted from the round-trip time, wherein the transmission control unit defines the reception terminals having similar time variations in the bit rate among the receiving terminals as a small group, and determines a transmission interval of the probe packets and the bit rate of the content data to be transmitted collectively for the receiving terminals belonging to the same small group.
In order to achieve the foregoing object, the content distribution method according to the present invention is a content distribution method used for a content distribution system constituted with a content distribution device and a plurality of receiving terminals connected mutually via a network and the content distribution device transmits content data to the receiving terminals, wherein: a content packet transmission unit of the content distribution device transmits the content data to the receiving terminals; a probe packet transmission unit of the content distribution device transmits probe packets for estimating a state of the network to the receiving terminals; a probe packet reception unit of the content distribution device receives return packets for the probe packets from the receiving terminals; a transmission control unit of the content distribution device specifies round-trip time from transmission of the probe packets to reception of the return packets; the transmission control unit of the content distribution device defines the receiving terminals having similar time variations in the bit rate among the receiving terminals as a small group; and the transmission control unit of the content distribution device determines a transmission interval of the probe packets and the bit rate of the content data to be transmitted collectively for the receiving terminals belonging to the same small group.
In order to achieve the foregoing object, the content distribution program according to the present invention is a content distribution program used in a content distribution system which is constituted with a content distribution device and a plurality of receiving terminals connected mutually via a network and the content distribution device transmits content data to the receiving terminals. The program is characterized to cause a computer provided to the content distribution device to execute: a procedure for transmitting the content data to the receiving terminals; a procedure for transmitting probe packets for estimating a state of the network to the receiving terminals; a procedure for receiving return packets for the probe packets from the receiving terminals; a procedure for specifying round-trip time from transmission of the probe packets to reception of the return packets; a procedure for determining the bit rate of the content data to be transmitted from the specified round-trip time; a procedure for defining the receiving terminals having similar time variations in the bit rate among the receiving terminals as a small group; and a procedure for determining a transmission interval of the probe packets collectively for the receiving terminals belonging to the same small group.
The present invention is structured to put the receiving terminals having similar time variations in the bit rate (i.e., terminal of same bottle-neck link) into a small group as described above. Therefore, the number of probe packets to be transmitted to the same small group can be reduced. This makes it possible to provide a content distribution system, a content distribution device, a content distribution method, and a program thereof, which can reduce the number of probe packets to be transmitted towards the receiving terminals from the distribution device while properly controlling the bit rate for each of the receiving terminals and thereby can suppress the reduction in the number of streams that can be distributed simultaneously as well as the deterioration in the communication quality.